[AArch64][NFC] Rewrite aarch64-reassociate-accumulators tests

In this patch I've made two changes:

1. The aarch64-reassociate-accumulators-sve.ll test wasn't
representative of loop vectorised code because the pointers
were incrementing by a fixed value and so were only valid
when vscale=1. I've changed this to increment the pointer
by the appropriate scalable amount.
2. I've generated the CHECK lines for both the NEON and SVE
test using filters to make the tests more maintainable.

Author: david-arm